This invention relates to x-ray telescopes and more particularly to variable magnification ultra-high spectral resolution stigmatic glancing incidence x-ray telescopes capable of simultaneously producing multiple high spatial and ultra-high spectral resolution images of solar and stellar sources at numerous well defined spectral wavebands.
For applications of obtaining ultra-high spatial resolution observations with high sensitivity detectors, such as CCD's or Multi-Anode MicroChannel Arrays (MAMA'S), variable magnifications are highly desirable. For maximum information of plasma diagnostics, ultra-high spectral resolution two dimensional x-ray/extreme ultraviolet images are very important. However, this capability does not at present exist. Very high resolution telescopes, such as the optical system currently under development for the Advanced X-Ray Astrophysics Facility (AXAF) have a fixed focal length and fixed field of view as dictated by the fundamental parameters of the primary mirror. These telescopes can perform spectroscopy of point sources but are extremely limited when performing simultaneous high resolution spectrography and imaging of extended sources. They have been designed with the greatest emphasis placed upon the harder rather than the softer components of the x-ray spectrum.
The ability to produce images of sources at x-ray energies up to 10 keV is of profound significance to the solution of many of the most important problems of astrophysics and solar physics. An instrument for simultaneously performing high spatial resolution images of the sun and of astrophysical sources at numerous well defined spectral wavebands is disclosed in applicant's copending application (Ser. No. 756,979) filed on Aug. 15, 1985, entitled Multispectral Glancing Incidence X-Ray Telescope. In that application a telescope system was disclosed which made high resolution and magnification imaging of solar and stellar x-ray and extreme ultraviolet radiation possible. The telescope system there disclosed images over a broad band of hard x-ray and extreme ultraviolet radiation, in the range of 30 angstroms and below using Wolter type optics without increasing the physical size of the telescope. This was accomplished by combining ellipsoidal layered synthetic microstructure (LSM) mirrors operating at inclined orientations in combination with a glancing incidence Wolter I system with off-axis x-ray detector means with the LSM optics positioned behind the primary focus of the Wolter I primary mirrors system, the LSM mirrors being concave and positioned behind the primary focus of the Wolter I primary mirror system. The apparatus therein disclosed thus made it possible to obtain high spatial and spectral resolution images of point sources or of extended sources of x-ray emission at wavelengths shorter, i.e., higher energies, than could be imaged with the spectral slicing x-ray telescope disclosed in applicant's earlier U.S. Pat. No. 4,562,583 dated Dec. 31, 1985, which operated at normal incidence with all optical elements positioned on the optical axis.
Layered synthetic microstructure (LSM) coatings have during the past few years come to be more commonly called "multilayer coatings" or simply "multilayers", and hence the more modern terminology will be used in the present application.
In the prior art, Wolter x-ray telescopes have been used with single or nested mirrors to focus x-rays from astronomically distant point or extended sources. These telescopes use x-ray mirrors which operate at a glancing or grazing angle of incidence. The mirrors may be used uncoated or may be coated with a high-Z material such as gold, platinum or iridium. The solar x-ray telescopes which were flown on SKYLAB operated at grazing angles of 54 arc minutes and could effectively reflect only x-rays of energies lower than the 0.5 keV (wavelengths&gt;6 angstroms). These Wolter Type I mirrors use internally reflecting, coaxial and confocal paraboloidal and hyperboloidal mirrors. Astrophysical telescopes, such as HEAO, XMM and AXAF, have been designed to operate at glancing angles in the range of 20 to 50 arc minutes, making it possible for them to focus and image x-rays with energies up to 8 to 10 keV (wavelengths &gt;1.2 angstroms). Images with these systems are typically recorded on high resolution photographic film or other solid-state or gas filled detectors such as CCD's Position Sensitive Proportional Counters, Multi-Anode Micro-Channel Arrays (MAMAS). Techniques for coupling Wolter telescopes to solid state detectors by means of convex hyperboloid mirrors were described in the aforesaid U.S. Pat. No. 4,562,583. However, this device is not capable of operating over the entire wavelength range which can be covered by glancing incidence x-ray telescopes due to the difficulty of fabricating Layered Synthetic Microstructure (LSM) coatings capable of operating at wavelengths significantly less than 30 angstroms when cofigured at normal incidence.
Some spectral information has been achieved by means of bandpass filters placed in front of the prime focus of glancing incidence telescopes, as on ATM Experiments S-054 and S056 which were flown by NASA on its first orbiting space station, SKYLAB. However, this technique provides very crude, low spectral resolution filtergrams which do not have adequate spectral resolution for proper diagnostics of the solar or of stellar plasmas. Grating spectroscopy instruments were also flown on SKYLAB for extreme ultraviolet spectroscopy, but these instruments were not capable of functioning at x-ray wavelengths below 171.ANG. and had very low sensitivity below 304.ANG.. However, the information produced was of crucial importance for solar x-ray plasma diagnostics.
The primary disadvantages of using an x-ray telescope with filters to produce spectral data is that the bandpasses are so wide as to encompass tens, hundreds or even thousands of spectral lines resulting from plasma in the atmosphere of the sun or any stellar source. The emission lines originate in plasmas at vastly varying temperature and emanating from widely differing heights in the solar or stellar atmosphere.
In the applicant's copending application Ser. No. 756,979 entitled Multispectral Glancing Incidence X-Ray Telescope, a system was disclosed having the capability of obtaining high resolution images in different spectral bands over the entire wavelength range that the glancing incidence primary optic was capable of reflecting (1.ANG.-100.ANG.). Disclosed in that application was a high resolution x-ray telescope having a rotatable cylindrical carrier on which a plurality of concave mirrors were mounted, the mirrors being coated with different coatings, and the carrier being rotated to place a selected mirror in the path of the reflected incoming beam to obtain high resolution images of different wavelengths dependent upon which mirror was selected. Even that instrument only provides high spectral resolution images, with the bandpasses determined by the spectral bandpass of the multilayer coating of the ellipsoidal optic. In some regions of the solar atmosphere, a bandpass of only a few angstroms may include many spectral lines from low temperature plasma located in the upper chromosphere or transition region combined with emission from spectral lines from high temperature plasma from the solar corona. During the Oct. 23, 1987 flight of the Stanford/MSFC Rocket X-Ray Telescope, in which we produced the first high resolution, full disk x-ray images of the sun with multilayer x-ray optics (Science, Vol. 241, 1725-1868), the 171-175.ANG. images are dominantly produced by Fe IX (171.075.ANG.) and Fe X (175.534.ANG.) emission at 1 million degrees, but those images are contaminated by some undefined low intensity component of emission at 500,000 degrees due to the presence of lower temperature emission from O V (172.174.ANG.) and the O VI doublet (172.936.ANG. and 173.081.ANG.) from the plasma in the cooler transition region. As an example of the complexity of the solar atmosphere, it should be noted that within the narrow (171-176.ANG.) bandpass of that Cassegrain multilayer x-ray telescope, there exists 21 different spectral emission lines from several different ionization states of Iron, Nickel and Oxygen. At the shorter wavelengths, the number of closely adjacent spectral lines from diverse ionization states becomes even more acute. These pictures of the sun are the first images to show the presence of the solar network (super-granulation) structure at coronal temperatures. However, that important discovery is somewhat confused by the presence of the lower temperature Oxygen lines in the instrument bandpass. Even though those lines are believed to be sufficiently weak to have produced a non-observable contribution to the images their exact contribution must await further studies.